A device for acquiring three dimensional data of a target surface or other object is very essential for target recognition in variety of fields e.g. robotics, IC inspection etc. Such a device can transmit the acquired three dimensional data to other instruments for tool manipulation, data inputting to computers for their analytical judgement etc.
A number of techniques have been used in the past to generate three dimensional information and a few typical ones are stereo vision, laser radar and triangulation technique. The stereo vision requires accurate opto-mechanical alignment and complex data computation, resulting in slow response, low sensitivity and resolution etc. The laser radar technique makes use of the time-of-flight measurement of an optical pulse. Because it requires better than 0.1 pico seconds of time delay resolution to measure the height of 0.001" in indoor applications. The system is at present far from practical.
Triangulation, on the other hand, is perceived as the best overall performance method for variety of applications. Among the triangulation techniques, a three dimensional raster scanned technique has shown excellent results. U.S. Pat. No. 4,627,734 Dec. 9, 1986 Rioux describes such optical three dimensional imaging system utilizing a laser scanner with a synchronously scanned position sensitive detector. In the patented system, the projected beam and the detected beam are scanned synchronously so that the detected position in the position sensitive detector remains unchanged when the beams scan a reference surface, the shape and location of which are uniquely determined by the optical parameters of the system. The distance of the detector from the reference surface can be set arbitrarily. The detected position in the detector only changes when the beam is reflected from a surface point that is either nearer to or further from the reference surface.
Similar synchronous scanning techniques are disclosed in U.S. Pat. No. 4,171,917 Oct. 23, 1979 Pirlet. The patent uses a pair of synchronously rotating polygonal mirrors, one for scanning the projected beam and another for receiving the scattered beam, for determining the profile of a surface of an object.
U.S. Pat. No. 4,170,398 Oct. 9, 1979, Koester discloses another triangulation technique which employes a pair of synchronously rotating flat mirrors to scan the surface and to detect scattered beam. The sanning technique of the patent is for scanning microscope and in particular for viewing biological tissues at varied angles.
One of the common problems of the triangulation process is its non-linearity along the scanning line in that the reference surface with respect to which the height of the target surface is measured is not planar but rather spherical, cylindrical or other curved surface, depending upon the optical arrangement of the system used.
The present invention as one of its applications obviates such non-linearity by the use of unique optical configurations.
It should also be noted that U.S. Pat. No. 4,009,829 July 11, 1978, Sraayer suggests the use of a single converging meniscus lens in the optical path between the scanner and the flat field surface focussing the diverging light beam as a point at all locations on the flat field. Of course, this optical arrangement is not a synchronous scanning and only the focussing of the projected scanning beam is considered. While this avenue has been tried for synchronous scanning it has been shown to be impractical.